There's no "real" story for this level yet (other than Vader is at a Rebel base with a bunch of Stormtroopers). The script will be written around the level, this is how I used to make my MotS levels and I generally enjoyed doing them much more this way than writing the script first (a main reason that "Party Crasher" failed).

Check out this thread at Massassi for some of the ideas that have been thrown around for this level: